This application claims the priority of German application 97 11 207.2, filed Mar. 18, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a drawing device for a press, and more particularly, to a drawing device having a drawing slide, having a sheet holder with a sheet held in it, having a pressure cheek which is arranged under the sheet holder and which is connected with a piston rod which can be displaced in parallel to the drawing direction and which is operatively connected with a preacceleration piston of a preacceleration cylinder, and drawing cylinders which are applied to the pressure cheek by way of drawing rods, the drawing cylinders having drawing pistons with first and second piston spaces, the first piston space directed to the drawing rod being pressure controlled during the drawing operation for the sheet forming.
DE-OS 36 40 788 describes a drawing device with a preacceleration cylinder and preacceleration piston for preaccelerating, the pressure cheek before the start of the drawing operation in the drawing direction to the speed of the drawing slide. The subsequent sheet forming takes place by a targeted pressure control in the drawing cylinders. For this purpose, in a first piston space of a drawing cylinder directed to the pressure cheek of the drawing device, a correspondingly controlled counterpressure is in each case applied which acts oppositely to the drawing direction.
The known drawing device has a disadvantage in that, when the press is switched off during a drawing operation, the drawing pistons of the drawing cylinders will also stop. Although pressure medium is still situated in the piston spaces of the drawing cylinders, because no more displacement takes place during the machine stoppage, no pressure control also can take place. Simultaneously, however, a slow pressure reduction necessarily takes place in the piston spaces, for example, because of leakages. The resulting pressure change in the system becomes larger, the longer the machine stoppage lasts. This means that the sheet metal parts situated in the press, when the press is restarted, have to be removed from the press as scrap because of the uncontrolled forming as the results of changed pressures.
It is, therefore, an object of the present invention to improve a drawing device such that, also during a stoppage of the press, particularly in the case of an emergency stop, after the restart, the same conditions will exist as at the beginning of the press stoppage so that no scrap has to be removed but, on the contrary, the interrupted sheet forming can immediately take place under the same conditions.
According to the present invention, this object has been achieved by providing that in the case of a press stoppage during a sheet forming, a control valve device can be activated by means of which, for maintaining the pressure existing in the first piston space at the start of the press stoppage, during the time of the press stoppage, pressure medium can be charged into the first piston space by means of a pressure medium generator.
The control according to the present invention by way of the control valve device, which as a type of emergency control, provides during a press stoppage that pressure losses in the respectively first piston space of the drawing cylinders are compensated by a correspondingly controlled addition of pressure medium, ensures that, at the restart of the press, the same pressure exists as at the time of the press stoppage. This means that the interrupted sheet forming can be continued under the same conditions and at the same point so that no scrap parts are created.
The control according to the invention is achieved by relatively simple supplementary parts. This applies particularly if a pressure medium generator, such as the pump, which exists anyhow in the operation of the drawing device, is used as the pressure medium generating device and activates the preacceleration piston of the preacceleration cylinder.
In a simple manner, the control valve device can be constructed as an electrovalve device which is activated in an arbitrary manner by a control command when a press stoppage occurs.
It may advantageously be provided in this case that a path-dependent valve device is arranged between the pressure generator and the control valve device. As the result of this measure, the activation of the control valve device can be carried out in a targeted manner only in the case of a press stoppage when its activation is required and therefore also causes no damage. Thereby, the activation will be carried out only when drawing rods of the drawing pistons are situated in the position in which a sheet forming takes place because of a pressure caused by the drawing slide. If no counterpressure exists as the result of the drawing slide, which is the case, for example, in the upper position of the drawing device, the control valve device is not activated.
In a constructionally simple manner, the path-dependent valve device can be applied to the piston rod of the preacceleration piston by a sensing element, because of the piston rod taking up its respective position; in particular, the piston rod takes over the restoring position of the pressure cheek upwards after a sheet forming has taken place. In other words, the position of the piston rod can be used in a simple manner for controlling the control valve device.
As the sensing element, a roller tappet can, for example, be provided which rolls along a correspondingly shaped curved path on the circumference of the piston rod. For detecting the pressure in the respectively first piston space of the drawing cylinders and its control, a pressure sensor and a proportional valve can be used.